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At least 775 records · Page 43Linked to original sources
PubMed2006

Vector-velocity estimation in swept-scan using a K-space approach.

The swept-scan technique (i.e., continuously moving a single-crystal transducer during pulse-echo data acquisition) is used in high-frequency, ultrasonic flow imaging. Relative to the conventional step-scan technique, swept scanning improves the rate of data acquisition and enables near-real-time, high-frequency color flow mapping. However, the continuous transducer movement may have non-negligible effects on accuracy of velocity estimation. This paper introduces a spatial frequency domain (i.e., k-space) approach that quantifies the effects of both lateral and axial motions in a swept scan. It is shown that the k-space representation is equivalent to a Doppler-radio frequency (RF) frequency domain representation, and that transducer movement in the swept-scan technique results in a change in Doppler bandwidth. In addition, a vector velocity estimator is developed based on the proposed k-space approach. Both simulations and flow-phantom experiments were performed to evaluate the performance of the proposed vector velocity estimator. A 45-MHz transducer was scanned at 20 mm/s. The Doppler angle ranged from 29 degrees to 90 degrees, and the flow velocities ranged from 15 to 30 mm/s. The results show that the proposed k-space vector velocity estimator exhibited a mean error of 2.6 degrees for flow-direction estimation, with the standard deviation ranging from 2.2 degrees to 8.2 degrees. In comparison, for the conventional spectral-broadening-based vector velocity estimator ignoring the swept-scan effect, the mean error became 15 degrees and the standard deviations were from 2.7 degrees to 6.6 degrees.

Algorithms↗
PubMed2004

Consequence modeling using the fire dynamics simulator.

The use of Computational Fluid Dynamics (CFD) and in particular Large Eddy Simulation (LES) codes to model fires provides an efficient tool for the prediction of large-scale effects that include plume characteristics, combustion product dispersion, and heat effects to adjacent objects. This paper illustrates the strengths of the Fire Dynamics Simulator (FDS), an LES code developed by the National Institute of Standards and Technology (NIST), through several small and large-scale validation runs and process safety applications. The paper presents two fire experiments--a small room fire and a large (15 m diameter) pool fire. The model results are compared to experimental data and demonstrate good agreement between the models and data. The validation work is then extended to demonstrate applicability to process safety concerns by detailing a model of a tank farm fire and a model of the ignition of a gaseous fuel in a confined space. In this simulation, a room was filled with propane, given time to disperse, and was then ignited. The model yields accurate results of the dispersion of the gas throughout the space. This information can be used to determine flammability and explosive limits in a space and can be used in subsequent models to determine the pressure and temperature waves that would result from an explosion. The model dispersion results were compared to an experiment performed by Factory Mutual. Using the above examples, this paper will demonstrate that FDS is ideally suited to build realistic models of process geometries in which large scale explosion and fire failure risks can be evaluated with several distinct advantages over more traditional CFD codes. Namely transient solutions to fire and explosion growth can be produced with less sophisticated hardware (lower cost) than needed for traditional CFD codes (PC type computer verses UNIX workstation) and can be solved for longer time histories (on the order of hundreds of seconds of computed time) with minimal computer resources and length of model run. Additionally results that are produced can be analyzed, viewed, and tabulated during and following a model run within a PC environment. There are some tradeoffs, however, as rapid computations in PC's may require a sacrifice in the grid resolution or in the sub-grid modeling, depending on the size of the geometry modeled.

Computer Simulation↗
PubMed2003

Theory and experiment of Fourier-Bessel field calculation and tuning of a pulsed wave annular array.

A one-dimensional (1D) Fourier-Bessel series method for computing and tuning (beamforming) the linear lossless field of flat pulsed wave annular arrays is developed and supported with both numerical simulation and experimental verification. The technique represents a new method for modeling and tuning the propagated field by linking the quantized surface pressure profile to a known set of limited diffraction Bessel beams propagating into the medium. This enables derivation of an analytic expression for the field at any point in space and time in terms of the transducer surface pressure profile. Tuning of the field then also follows by formulating a least-squares design for the transducer surface pressure with respect to a given desired field in space and time. Simulated and experimental results for both field computation and tuning are presented in the context of a 10-ring annular array operating at a central frequency of 2.5 MHz in water.

Algorithms↗
PubMed2001

Non-Hamiltonian equations of motion with a conserved energy.

In 1980 Andersen introduced the use of "extended system" as a means of exploring by molecular dynamics simulation the phase space of a physical model according to a desired ensemble distribution different from the standard microcanonical function. Following his original work on constant pressure-constant enthalpy a large number of different equations of motion, not directly derivable from a Hamiltonian, have been proposed in recent years, the most notable of which is the so-called Nosé-Hoover formulation for "canonical" molecular dynamics simulation. Using a generalization of the symplectic form of the Hamilton equations of motion we show here that there is a unique general structure that underlies most, if not all the equations of motion for "extended systems." We establish a unifying formalism that allows one to identify and separately control the conserved quantity, usually known as the "total energy" of the system, and the phase-space compressibility. Moreover, we define a standard procedure to construct conservative non-Hamiltonian flows that sample the phase space according to a chosen distribution function [Tuckerman et al., Europhys. Lett. 45, 149 (1999)]. To illustrate the formalism we derive new equations of motion for two example cases. First we modify the equations of motion of the Nosé-Hoover thermostat applied to a one-dimensional harmonic oscillator, and we show how to overcome the ergodicity problem and obtain a canonical sampling of phase space without making recourse to additional degrees of freedom. Finally we recast an idea recently put forward by Marchi and Ballone [J. Chem. Phys. 110, 3697 (1999)] and derive a dynamical scheme for sampling phase space with arbitrary statistical biases, showing as an explicit application a demixing transition in a simple Lennard-Jones binary mixture.

Journal Article↗
PubMed2002

Parallel-in-time molecular-dynamics simulations.

While there have been many progress in the field of multiscale simulations in the space domain, in particular, due to efficient parallelization techniques, much less is known in the way to perform similar approaches in the time domain. In this paper we show on two examples that, provided we can describe in a rough but still accurate way the system under consideration, it is indeed possible to parallelize molecular dynamics simulations in time by using the recently introduced pararealalgorithm. The technique is most useful for ab initio simulations.

Journal Article↗
PubMedDate not supplied

[The ability for psych self-regulation as a factor in resistance to the stresses in extreme conditions of space flight].

Significance of the ability for psych self-regulation in the context of resistance to the stresses of space flight was studied in an experiment with 9 test-subjects simulating such factors of space flights as 8 to 9-day isolation and confinement, some physiological effects of microgravity in a head-down position at -8 degrees for 7 d, artificial climate, and implementation of dock and piloting operations. Stress resistance, self-regulation, mental performance and behavior were assessed with the use of computerized tests "Mirror coordinograph", "Relaxometer", and "SOPR-monitoring". The ability to voluntary control psych was shown to be favorable to stress-resistance and rapid recovery of mental efficiency after the natural decline in consequence of the experimental simulation. The ability for psych self-regulation is one of the major criteria of professional selection for exposure to extreme conditions.

Adaptation, Psychological↗
PubMedDate not supplied

Psychological, psychiatric, and interpersonal aspects of long-duration space missions.

America's future in space calls for manned missions that are of long duration and increasing complexity. Under these conditions, psychological and interpersonal stressors will take on added importance in affecting the safely of the crew and the outcome of the mission. Through an analysis of reports from manned American and Soviet space missions and Earth-bound simulations, several psychological, psychiatric, and interpersonal issues can be identified that could affect the success of the space station and other long-duration space ventures. Psychological issues include sleep problems, alteration in time sense, demographic effects, career motivation, transcendent experiences, homesickness, and alteration in perceptual sensitivities. Psychiatric issues include anxiety, depression, and psychosis, psychosomatic symptoms, emotional problems related to the stage of the mission, and postflight personality changes. Interpersonal issues include interpersonal tension, decreased cohesiveness over time, need for privacy, and task vs emotional leadership. Steps can be taken to minimize the impact of these issues, both before and during the mission.

Aerospace Medicine↗
PubMed2006

MR thermometry characterization of a hyperthermia ultrasound array designed using the k-space computational method.

BACKGROUND: Ultrasound induced hyperthermia is a useful adjuvant to radiation therapy in the treatment of prostate cancer. A uniform thermal dose (43 degrees C for 30 minutes) is required within the targeted cancerous volume for effective therapy. This requires specific ultrasound phased array design and appropriate thermometry method. Inhomogeneous, acoustical, three-dimensional (3D) prostate models and economical computational methods provide necessary tools to predict the appropriate shape of hyperthermia phased arrays for better focusing. This research utilizes the k-space computational method and a 3D human prostate model to design an intracavitary ultrasound probe for hyperthermia treatment of prostate cancer. Evaluation of the probe includes ex vivo and in vivo controlled hyperthermia experiments using the noninvasive magnetic resonance imaging (MRI) thermometry. METHODS: A 3D acoustical prostate model was created using photographic data from the Visible Human Project. The k-space computational method was used on this coarse grid and inhomogeneous tissue model to simulate the steady state pressure wavefield of the designed phased array using the linear acoustic wave equation. To ensure the uniformity and spread of the pressure in the length of the array, and the focusing capability in the width of the array, the equally-sized elements of the 4 x 20 elements phased array were 1 x 14 mm. A probe was constructed according to the design in simulation using lead zerconate titanate (PZT-8) ceramic and a Delrin plastic housing. Noninvasive MRI thermometry and a switching feedback controller were used to accomplish ex vivo and in vivo hyperthermia evaluations of the probe. RESULTS: Both exposimetry and k-space simulation results demonstrated acceptable agreement within 9%. With a desired temperature plateau of 43.0 degrees C, ex vivo and in vivo controlled hyperthermia experiments showed that the MRI temperature at the steady state was 42.9 +/- 0.38 degrees C and 43.1 +/- 0.80 degrees C, respectively, for 20 minutes of heating. CONCLUSION: Unlike conventional computational methods, the k-space method provides a powerful tool to predict pressure wavefield in large scale, 3D, inhomogeneous and coarse grid tissue models. Noninvasive MRI thermometry supports the efficacy of this probe and the feedback controller in an in vivo hyperthermia treatment of canine prostate.

Acoustics↗
PubMed2004

Estimation of the density of states by multicanonical molecular dynamics simulation.

We formulate a procedure for calculating the density of states (DOS) from a multicanonical molecular dynamics (MMD) simulation. DOS cannot be obtained directly from the result of MMD simulation, because the Gaussian thermostat that is used in MMD simulation restricts the system to a spherical surface in momentum space. We perform MMD simulation for liquid Ar with Lennard-Jones potentials and evaluate DOS. Some physical quantities are estimated as a function of temperature from that DOS. The internal energy, entropy, and Helmholtz free energy are in good agreement with experiment. The quantity related to the fluctuation--the specific heat at constant volume--does not agree with experiment, which is ascribed to insufficient accuracy of DOS.

Journal Article↗
PubMed1992

A model of NMDA receptor-mediated activity in dendrites of hippocampal CA1 pyramidal neurons.

1. The role of synaptic activation of NMDA (N-methyl-D-aspartate) receptor-mediated conductances on CA1 hippocampal pyramidal cells in short-term excitability changes was studied with the use of a computational model. Model parameters were based on experimental recordings from dendrites and somata and previous hippocampal simulations. Representation of CA1 neurons included NMDA and non-NMDA excitatory dendritic synapses, dendritic and somatic inhibition, five intrinsic membrane conductances, and provision for activity-dependent intracellular and extracellular ion concentration changes. 2. The model simulated somatic and dendritic potentials recorded experimentally. The characteristic CA1 spike afterdepolarization was a consequence of the longitudinal spread of dendritic charge, reactivation of slow Ca(2+)-dependent K+ conductances, slow synaptic processes (NMDA-dependent depolarizing and gamma-aminobutyric acid-mediated hyperpolarizing currents) and was sensitive to extracellular potassium accumulation. Calcium currents were found to be less important in generating the spike afterdepolarization. 3. Repetitive activity was influenced by the cumulative activation of the NMDA-mediated synaptic conductances, the frequency-dependent depression of inhibitory synaptic responses, and a shift in the potassium reversal potential. NMDA receptor activation produced a transient potentiation of the excitatory postsynaptic potential (EPSP). The frequency dependence of EPSP potentiation was similar to the experimental data, reaching a maximal value near 10 Hz. 4. Although the present model did not have compartments for dendritic spines, Ca2+ accumulation was simulated in a restricted space near the intracellular surface of the dendritic membrane. The simulations demonstrated that the Ca2+ component of the NMDA-operated synaptic current can be a significant factor in increasing the Ca2+ concentration at submembrane regions, even in the absence of Ca2+ spikes. 5. Elevation of the extracellular K+ concentration enhanced the dendritic synaptic response during repetitive activity and led to an increase in intracellular Ca2+ levels. This increase in dendritic excitability was partly mediated by NMDA receptor-mediated conductances. 6. Blockade of Ca(2+)-sensitive K+ conductances in the dendrites increased the size of EPSPs leading to a facilitation of dendritic and somatic spike activity and increased [Ca2+]i. NMDA receptor-mediated conductances appeared as an amplifying component in this mechanism, activated by the relatively depolarized membrane potential. 7. The results suggest that dendritic NMDA receptors, by virtue of their voltage-dependency, can interact with a number of voltage-sensitive conductances to increase the dendritic excitatory response during periods of repetitive synaptic activation. These findings support experimental results that implicate NMDA receptor-mediated conductances in the short-term response plasticity of the CA1 hippocampal pyramidal neuron.

Animals↗
PubMed2000

Simulated pathline visualization of computed periodic blood flow patterns.

Improvements in computer hardware and software have made it possible to model pulsatile blood flow in realistic arterial geometries. Such studies produce enormous amounts of velocity data, which are often difficult to interpret and communicate using traditional contour and/or vector field plots. Inspired by in vitro flow visualization techniques such as particle image velocimetry (PIV), we describe a simple and effective method for visualizing periodic three-dimensional velocity data, based on the subdivision and sequential display of computed particle trajectories. Analogous to a PIV experiment, the length and spacing of such simulated particle pathlines are controlled by user-specified shutter-speed and frame rate variables. Strategies for color-coding pathlines to highlight important hemodynamic features such as recirculation zones and branch flow division are presented.

Biomechanical Phenomena↗
PubMed2006

The MATROSHKA facility--dose determination during an EVA.

On 29 January 2004 the MATROSHKA facility was launched with a Russian Progress to the International Space Station. MATROSHKA is an ESA project, which has been achieved under the direction of the German Aerospace Center (DLR). The project is a cooperation of >16 research institutes from all over the world and is currently the biggest international experiment in radiation dosimetry ever performed in space. The facility simulates, as exact as possible, an astronaut during an extravehicular activity. It was successfully installed outside the Russian segment 'Zvezda' on 26 February 2004 and will remain there for a 1.5 year exposure period. The main task of the facility is to measure particle fluence and energy spectra, dose and dose rates outside and inside--including organ dose determination--in an anthropomorphic phantom mounted on the outside of the Space Station with passive and active dosemeter systems.

Body Burden↗
PubMed2000

Responses of muscle sympathetic nerve activity to static handgrip exercise after 14 days of exposure to simulated microgravity.

Decrease in muscle perfusion affects on cardiovascular response to exercise. Muscle hypoperfusion enhances the increase in blood pressure responses to exercise. Muscle perfusion depends not only on central blood pressure but also how fit the active muscle is above or below the heart level; muscle perfusion decreases as arm is elevated. Static exercise increases muscle sympathetic nerve activity (MSNA) innervating vessels in non-active muscles. The exercise-induced increase in MSNA is mainly mediated by stimulating chemosensitive muscle afferents in active muscles. However, the effect of arm elevation on MSNA during forearm exercise is not examined. On the other hand, space flight and simulated microgravity exposure causes reduction in muscle blood flow, suggesting chronic muscle hypoperfused condition during simulated microgravity. Therefore, there is a possibility that arm elevation after microgravity exposure alters MSNA responsiveness during exercise. However, arm elevation effect after exposure to simulated microgravity is not examined.

Arm↗
PubMed2005

A directed essential dynamics simulation of peptide folding.

We present a directed essential dynamics (DED) method for peptide and protein folding. DED is a molecular dynamics method based on the essential dynamics sampling and the principal component analysis. The main idea of DED is to use principal component analysis to determine the direction of the most active collective motion of peptides at short intervals of time (20 fs) during the folding process and then add an additional force along it to adjust the folding direction. This method can make the peptides avoid being trapped in the local minima for a long time and enhance the sampling efficiency in conformational space during the simulation. An S-peptide with 15 amino acids is used to demonstrate the DED method. The results show that DED can lead the S-peptide to fold quickly into the native state, whereas traditional molecular dynamics needs more time to do this.

Biophysics↗
PubMed2003

Simulations for inverse radiation therapy treatment planning using a dynamic MLC algorithm.

The inverse radiation treatment planning model for a dynamic multileaf collimator (MLC) is used to find the optimal solution of planning problem. The model for dynamic MLC is explained in Tervo et al (2003 Appl. Math. Comput. 135 227-50). The advantage of this model is that it optimizes leaf velocity parameters directly. Our algorithm uses a gradient-based local optimization method. Two patient cases, prostate carcinoma and tonsilla carcinoma, are studied. Field arrangements are pre-selected and velocity parameters for MLC leaves are optimized to obtain the prescribed dose in the patient space. In both simulated cases, high dose distribution conforms the planning target volume well and organs-at-risk are saved in most parts. Simulations show that the model has its functionality in patient treatments, although it is still formal and needs further development.

Algorithms↗
PubMed2001

Dynamics and instability of electron phase-space tubes.

Two-dimensional simulations of beam-driven turbulence in the auroral ionosphere have shown the formation and instability of phase-space tubes. These tubes are a generalization of electron phase-space holes in a one-dimensional plasma. In a strongly magnetized plasma, such tubes vibrate at frequencies below the bounce frequency of the trapping potential. A theory for these vibrations yields quantitative agreement with kinetic simulations. Furthermore, the theory predicts that the vibrations can become unstable when resonantly coupled to electrostatic whistlers-also in agreement with simulations.

Journal Article↗
PubMed1995

Model for a pump that drives circulation of pleural fluid.

Physical and mathematical models were used to study a mechanism that could maintain the layer of pleural fluid that covers the surface of the lung. The pleural space was modeled as a thin layer of viscous fluid lying between a membrane carrying tension (T), representing the lung, and a rigid wall, representing the chest wall. Flow of the fluid was driven by sliding between the membrane and wall. The physical model consisted of a cylindrical balloon with strings stretched along its surface. When the balloon was inflated inside a vertical circular cylinder containing a viscous fluid, the strings formed narrow vertical channels between broad regions in which the balloon pressed against the outer cylinder. The channels simulated the pleural space in the regions of lobar margins. Oscillatory rotation of the outer cylinder maintained a lubricating layer of fluid between the balloon and the cylinder. The thickness of the fluid layer (h), measured by fluorescence videomicroscopy, was larger for larger fluid viscosity (mu), larger sliding velocity (U), and smaller pressure difference (delta P) between the layer and the channel. A mathematical model of the flow in a horizontal section was analyzed, and numerical solutions were obtained for parameter values of mu, U, delta P, and T that matched those of the physical model. The computed results agreed reasonably well with the experimental results. Scaling laws yield the prediction that h is approximately (T/delta P)(microU/T)2/3. For physiological values of the parameters, the predicted value of h is approximately 10(-3) cm, in good agreement with the observed thickness of the pleural space.

Body Fluids↗